1. Field of the Invention
The present invention relates to the field of digital image processing, and more particularly, to an apparatus and method for auto white balance control considering the effect of a single tone image, which can prevent erroneous auto white balance by taking into account the influence that a single tone image has on a gain computed for an auto white balance control.
2. Description of the Related Art
The human eye perceives objects as having the same color, even under different light sources. That is, the human eye perceives white as the same white tone even under different light sources with different color temperatures, such as the sun, fluorescent lamps and incandescent lamps. In contrast, image sensors employing solid state imaging devices convert light into electrical signals, thus they detect the color of an object differently depending on the light source illuminating the object. Therefore, most digital image processing devices (e.g., digital cameras) adopt auto white balance (AWB) control algorithms for automatically adjusting white balance in order to render the color of the object, detected by the image sensor, similar to the color perceived by the human eye.
In general, AWB control in digital image processing devices is carried out by representing a white object with a medium brightness level in an image as color signals (i.e., RGB signals) having identical output levels. As for a related art AWB control algorithm in wide use, assuming that the mean color value of an image is the value of white regardless of the variations of light sources, the mean color value is obtained from an image, and then the gain of a color signal level that allows the mean color value to be the value of white is obtained and applied to the image. For example, in the related art AWB control method, the values of Cb and Cr (hereinafter, referred to as ‘Cb and Cr values) are integrated and the mean values of the Cb and Cr values are calculated, thus adjusting the gains of red (R) and blue (B). Here, the Cb and Cr values refer to the color difference signals of pixels belonging to an area estimated at white in an input image.
However, those related art techniques for an AWB control have limitations, in that a single tone in an image affects the process of computing a white balance gain to a significant extent, thus resulting in defective white balance. Therefore, AWB control algorithms are required to adopt a technique that determines whether or not an input image is a single-tone image.
As for a known related art method of recognizing a single-tone image, the values of adjacent pixels are compared to one another with regard to the entirety of the RGB data of an input image and data about difference values between the adjacent pixels is accumulated, thus determining whether the input image is a single-tone image. This method, however, may impair system efficiency. Although capable of determining an ideal single-tone image through the comparison of adjacent pixels, this method is likely to cause errors at the final stage of determination if errors in the comparison between the adjacent pixels are accumulated continuously since color signals are concentrated around a specific area, not completely at the specific area. Moreover, comparing adjacent pixels to recognize a single-tone image may serve to prevent color distortion with regard to a single frame. However, this method fails to determine the transition from a composition image having the relatively uniform distribution of various colors to a single-tone image having the wide distribution of a single tone.
As for another known related art method of recognizing a single-tone image, assuming that the center of an image is an object area for camera capturing, the area of an outline determined to be an object in the corresponding area is determined and divided into blocks of a predetermined size, thus determining a single-tone image in due consideration of the level of dispersion and the mean value of brightness levels in the object area at the center of an image. This method, however, may increase system load due to the process of determining the area of an outline from the center of an image and dividing the area into blocks of a predetermined size. In the event that the assumption made regarding the object area is incorrect, this method may fail to acquire a desired result. Moreover, this method does not take into account the transition from a composite image to a single-tone image, thus it may result in erroneous results if such a transition takes place.